Quadrupole and Emission from Uranus: the Uranian Thermosphere,

We present an analysis of the near-infrared emission of Uranus, obtained from 1993 to 1995 at the United Kingdom Infrared Telescope (UKIRT) and the NASA Infrared Telescope Facility (IRTF). We report that in contrast to Jupiter and Saturn, prominent emission in the quadrupole and over2 3 tone and fundamental bands occurs globally. The rotational temperature and luminosity of and 2 3 were usually found to vary mildly with longitude. The and possibly the temperature appears to 2 3 vary with apparition, indicating a long-term variability of UranusÏs thermospheric structure. The thermosphere cooled between 1992 and 1995, while the ionosphere either cooled or otherwise su†ered a decline in the column of excited The luminosity in was observed as high as 1.6] 1010 W 3 H2(l\ 1) in 1993 May but remained within 10% of 1.0] 1010 W for four rotational phases observed in 1995 June near solar minimum. The luminosity in is predicted to be 10È20 times higher and less variable. H2(l\ 0) Similarly, the luminosity in was D1.1] 1011 W in 1995 June, half the value near solar maximum in 3 1992 April. The declining temperatures and luminosities may be induced by the declining phase of the solar cycle, when the far-ultraviolet/extreme-ultraviolet Ñux also declines. Although we have observed evidence of auroral emission by both molecular species, auroral processes apparently play only a secondary role in the observed excitation. The observed emission is consistent with an population in the 2 2 l\ 1 vibrational state in thermal equilibrium. But appears to deviate signiÐcantly from thermal 3 equilibrium in that the state is underpopulated. The central meridian intensity distribution is l2\ 2 2 qualitatively consistent with emission from a thick thermosphere. An important result is that the distribution of the fundamental-band emission di†ers from that of by having a pronounced concen3 2 tration toward the subsolar point. We propose that solar extreme-ultraviolet, Ðltered by UranusÏs H corona, is responsible for this concentration and is the dominant source of excited on the planet. 3 The K-band spectrum of UranusÏs rings indicates a nearly constant, featureless reÑectance over this band. Subject headings : infrared : solar system È planets and satellites : individual (Uranus)